Beneficial bacteria permanently colonize many body sites, with a growing appreciation for the importance of the microbiome to human health. Pioneering research has revealed that changes in gut bacteria impact metabolic and immunologic disorders such as obesity, inflammatory bowel disease (IBD) and multiple sclerosis (MS). Moreover, specific therapeutic bacterial molecules from the microbiome have been validated in experimental IBD and MS mouse models. Building on principles from the study of metabolism and immunity, reports have recently shown that the microbiome affects anxiety, nociception and aspects of brain development. These seminal studies may represent harbingers of extensive, currently undescribed, links between gut bacteria and the nervous system. Parkinson's disease (PD) results from neurodegeneration that leads to severe motor defects, with 3 million people worldwide suffering from this condition. Most cases are not hereditary; however, the contributions of environmental risk factors remain largely unknown. Based on the common occurrence of gastrointestinal (GI) symptoms and evidence supporting the hypothesis that neurodegeneration may initiate in the gut, examining a microbiome-brain connection in PD represents an exciting new frontier for research. Neurodegeneration in PD is believed to be caused by aggregation and/or accumulation of the prion-like protein, alpha-synuclein (alphaSyn). Disease symptoms can be modeled in mice by overexpression of human alpha-synuclein (Thy1-alphaSyn). To determine if the microbiome impacts disease, the first aim will test behavioral, cellular and functional features of disease in germ-free (gnotobiotic) Thy1-alphaSyn mice. Longitudinal analysis of disease progression will establish how gut bacteria contribute to neurodegeneration and aging. To test if changes in gut bacteria are relevant to PD, we will profile the microbiome of Thy1-alphaSyn mice using metagenomic (shotgun sequencing) and metatranscriptomic (RNAseq) analysis in the second aim. Differences between Thy1-alphaSyn and control mice may reveal specific microbes and microbial pathways that impact disease. Dopamine signaling is important for motor symptoms in PD, and the dopamine precursor L- DOPA is a mainline therapy. The final aim will employ a validated microfluidics approach to screen individual gut microbes for the potential to produce dopamine, and will test novel probiotic treatments in preclinical PD models. This project will investigate, for the first time, whether changes in gut bacteria affect the etiology of PD in mouse models. If successful, the key contribution of this project will be the transformative conceptual leap that PD may have a gut microbial origin, resulting in informed advances toward probiotic therapies for neurodegeneration.